1. Technical Field
The present invention relates to a digital radio transmitter system having a plurality of digital radio transmitters and more particularly to a device and a method for controlling a relative delay between analog radio frequency (R.F.) output signals of the plurality of digital radio transmitters.
2. Discussion of the Prior Art
Digital radio transmitter systems are an essential component of many modern communication systems like wideband systems for mobile communication. Generally, digital radio transmitter systems comprise one or more digital radio transmitters. A digital radio transmitter receives a digital input signal, converts the digital input signal into an analog signal and upconverts the analog signal to a n analog R.F. output signal. The analog R.F. output signal of the radio transmitter is passed through a power amplifier and then emitted from an antenna system.
An increasing number of applications require the use of digital radio transmitter systems with two or more radio transmitters, each radio transmitter defining a separate transmission branch. By equipping a digital radio transmitter system with a plurality of radio transmitters, applications like multi-carrier operation, radio transmitter diversity, load-sharing, and radio transmitter redundancy become feasible.
For example, during multi-carrier operation two or more carriers are jointly clipped in the digital domain of the digital radio transmitter system and then fed through separate radio transmitter branches. In other words, each carrier is upconverted to R.F. by a separate radio transmitter. After separate upconversion, the carriers are combined, jointly amplified and emitted from a single antenna. A typical example for a communication system using multiple carriers is a wideband code division multiple access (W-CDMA) system.
Another example for digital radio transmitter systems with two or more radio transmitter branches are systems operating according to the principle of radio transmitter diversity, of load-sharing or of radio transmitter redundancy. During these operation modes a single carrier is fed through two or more separate radio transmitters which operate in parallel. Each radio transmitter conducts a digital-analog conversion and then upconverts the carrier to R.F. The upconverted carriers outputted by the parallel radio transmitters are individually amplified and fed to individual antennas.
Of course, the concept of multi-carrier operation and the concepts of radio transmitter diversity, load-sharing or radio transmitter redundancy can be combined. As an example, the combination of 4-carrier operation and radio transmitter diversity necessitates eight separate radio transmitter branches which are configured such that the output signals of four radio transmitters are fed to one of two diversity antennas.
In digital radio transmitter systems with two or more digital radio transmitters, the individual radio transmitter branches ideally exhibit equal delay. In other words, if two digital signals have been concurrently fed into two different radio transmitters, the two corresponding analog R.F. output signals are ideally concurrently output. Thus, the absolute delays of two or more radio transmitter branches, i.e., the time difference between feeding an input signal into a radio transmitter and receiving the corresponding R.F. output signal from this radio transmitter, are ideally identical. This means in practice that the relative delay between the output signals of different radio transmitters, i.e., the time difference between outputting a first output signal by a first radio transmitter and outputting a corresponding second output signal by a second radio transmitter, must be as small as possible. For example, CDMA systems the relative delay must be small compared to the CDMA chip period.
If the relative delay increases, the transmission quality of a digital radio transmitter system with two or more radio transmitters decreases. In the case of multi-carrier operation, e.g., an increasing relative delay destroys the clipping effect. In the case of radio transmitter diversity, load-sharing or radio transmitter redundancy, an increasing relative delay results in self interference.
As has become apparent from the above, the object of maintaining a high transmission quality in a radio transmitter system with a plurality of radio transmitters necessitates equal or almost equal absolute delays within each radio transmitter branch to keep the relative delay between different radio transmitter branches as low a possible. However, equal or almost equal absolute delays are difficult to attain. One of the reasons therefore are the delay tolerances of the analog components like filters, amplifiers, etc. of individual radio transmitters. Exact synchronism between a plurality of radio transmitters is furthermore prevented by ambient temperature gradients, aging, replacement of radio transmitter boards in a single radio transmitter branch by boards with a different hardware version, etc.
There is, therefore, a need for a device which allows to control the relative delay between analog R.F. output signals of a plurality of digital radio transmitters in a digital radio transmitter system in an accurate and reliable manner to ensure a high transmission quality. There is also a need for a corresponding method for controlling the relative delay between analog R.F. output signals of a plurality of digital radio transmitters in a digital radio transmitter system.